Control circuit for a high voltage power supply utilizing a rectifier cascade

ABSTRACT

A high voltage power supply unit employs a control circuit for controlling the output voltage provided by way of a rectifier cascade. A highly ohmic resistance and a potentiometer having a movable tap are serially connected between a tap of the rectifier cascade and ground. The movable tap is connected to a difference amplifier by way of an emitter follower circuit and the difference amplifier is connected to control a high voltage generator which feeds the rectifier cascade. A power stage, in the form of a Darlington circuit, is interposed between the difference amplifier and the high voltage generator to provide a sufficiently high load current supply for the high voltage generator.

Forstmeyer Nov. 27, 1973 I CONTROL CIRCUIT FOR A HIGH VOLTAGE POWER SUPPLY UTILIZING A RECTIFIER CASCADE [75] Inventor: Udo Forstmeyer, Munich, Germany [73] Assignee: Siemens Aktiengesellschaft, Berlin,

Germany 22 Filed: Jan. 18, 1973 [21] Appl. No.1 324,747

[30] Foreign Application Priority Data UNITED STATES PATENTS Westwood 321/15 3,533,010 10/1970 Bowles 321/15X Primary Examiner-Gerald Goldberg Attorney-Benjamin 11. Sherman et a1.

[5 7 ABSTRACT A high voltage power supply unit employs a control circuit for controlling the output voltage provided by way of a rectifier cascade. A highly ohmic resistance and a potentiometer having a movable tap are serially connected between a tap of the rectifier cascade and ground. The movable tap is connected to a difference amplifier by way of an emitter follower circuit and the difference amplifier is connected to control a high voltage generator which feeds the rectifier cascade. A power stage, in the form of a Darlington circuit, is interposed between the difference amplifier and the high voltage generator to provide a sufficiently high load current supply for the high voltage generator.

1 Claim, 1 Drawing Figure CONTROL CIRCUITVFOR "A I HIGH "VOLTAGE POWER SUPPLY UTILIZING "AIRECTIFIER CASCADE BACKGROUND OF THE .lNVENTlQN means of a voltage dividerw andto feed only arportion I I of the outputvoltage as an actual value control :quantity to the control circuit. In the caseo'f'high voltage power supply units having an outputvoltage in the order of 18 kV,--this type of control faces considerable technical and economic difficulties. For example, the

resistances for 18 kV are.undesirablylargeand very,expensive. Since the l"8kV output should not be loaded to much, a high ohmic voltage dividerwith; a magnitude in the area of 1,000'MOhm must beused which, haddition, leads to an interference sensitive controlcircuit which easily tends tooscillate.

SUMMARY OF THE INVENTION It is the object of this invention to .provide-atcontrol circuit for controlling the output voltage'of-a'high voltage power supply nnit'which uses a conventional rectifier cascade, a so-called Greinacher circuit, forthe generation of high voltage,.and,which overcomes the disadvantages occu rring in conventional power supply units.

According to the invention, the foregoing object is achieved through the use of a circuit arrangement which includes a voltage divider consisting of a high ohmic resistance and a potentiometer connected between a tap of the rectifier cascade and-to ground. The connection to the rectifier cascade is preferably effected behind the first rectifier as viewed from the input of the rectifier cascade so that the tapped potential is considerably less than the output voltage. The potentiometer includes a movable contact which is connected, by way of an amplifier transistor for example, with an input of a difference amplifier of a low voltage power supply unit of conventional and generally known BRIEF DESCRIPTION OF THE DRAWING Other objects, features and advantages of the invention, its organization, construction and operation will be best understood from the following detailed description of a preferred embodiment of the .invention'taken. in conjunction with the accompanyingdrawing, on

which the single FIGURE is a schematic circuit representation of a high voltage powersupply constructed in :a rectifier cascade 'GK {Greinacher circuit) wherein the rectifier cascade :issupp'liedby' a'high voltage pulse generator "HG whichhas an input-connected to a'low voltage powerjsupply unit' 'consisting of a difference amplifier DV anda 'power stage DS.

5F or controlling {the output voltage A ofthe rectifier cascade GK, the: actual value control quantity could, as :is-thecase infthe conventional power supply;unit,be'directly diverted fromthe output voltage A. Since this technique,":however, leads'to the aforementioned technical and ".econornicdifficiilties due to the voltage A being in the order of l 8 theactual value control quantity is, according 'to the invention, derived "from a circuit point of *the rectifier cascade GK. This actual valuecontrol quantity'is considerably &less than the output voltage A and follows th'e output voltage A with a high'degree of'accu racy. A voltagedivider consisting of .a resistor R Land-a .potentiom'eter l is connected to a point *F-after the first rectifier within the rectifier cascade I GK, the point F "being particularly favorable according .to the aforementioned requirements. By connectingthe high ohmic resistance R1 to point F, an. ac-

tual value control quantity which is reduced -.to a rela- "tively small voltage value:is:available and is divided to ava'lue representative of the output voltage Aat the movable tapof the potentiometer P. The voltage at the movable-tap of the potentiometer P could now serve directly-as an input quantity for the differenceamplifier 13V. and therefore serve as a control voltage for-controlling IhBilItpllt voltage to the rectifier-cascade GK by -way 'o'f'the :power stage .DS and the high voltage pulse generator HG. Since, however, the difference amplifier "DVaccepts a different current in case of a different inputvoltage, thiswould lead to an imperfect representation of the output voltage at the movable tap of the potentiometer P. Forthis reason, an impedance transformer in the form of an emitter follower comprising a transistor T and a resistor R2 is connected between the input of the difference amplifier DV and the movable tapof the potentiometer 'P so that the potentiometer P onwthe one hand has a very high resistancetload and on the other hand the difference amplifier DV disposes of :a sufficiently high-load at the'emitter of the transistor T.'The collector ofthe transistor T is connected to the output of the difference amplifier DV and, conse-' quently, the transistor T isfed by'the battery voltage +UB-l of the difference amplifier DV. The desired value of the output voltage A,-i.e. the quoted 18 kV in this example, is adjusted by way of the potentiometer P. The second input of'the difference amplifier D'V,

which'is connected to ground, is positively -biased,-'in a well known'mannernot illustrated in the drawing, so that in case of a voltage increase at the output A .exceeding the'desiredvoltage-(P8 kV) changes the difference'voltage at the inputof the amplifier DV and thus the feed voltage of the rectifier cascade GK is -de-' creased'by' way of the power stage'DS' and thehighvoltage pulse generator HG. In case of an output voltage whichis to low, the same is increased by meansof the.

control circuit RS in the same manner;

However,the occasion might arise wherein an imperfection in the voltage divider, for example an imperfection in the resistor Rl,an imperfect solder joint or the break of a conductor, so that the current is interrupted in the voltage divider and that consequentlythe difference voltage at the input of the amplifier DV becomes insufficiently large over arlonger time. In this case, the output voltage would be strongly over controlled. ln order to avoid this situation, a Zener diode is connected'between the output of the difference amplifier with input pulses PA, high voltage output pulses which feed the rectifier cascade GK.

For increasing the initial voltage constant, as is illustrated in the drawing, the'low voltage power supply unit consisting of the difference amplifier DV and the sub sequently connected power stage DS, issupplied by two-voltage sources +UB1 and +UB2 having varying internal resistances. Therefore, the current supply of the two transistors of the difference amplifier DV takes place once directly by way ofthe voltage source +UB1 and at another time by way of the power stage DS by the voltage source +UB2. A capacitor C1 is connected between the movable tap of the potentiometer P and ground for smoothing of the control quantity which is tapped; at the potentiometer. i v

Thegcircuit according to the; present invention is not limited to the particular exemplary embodiment disclosed herein. The creation brute supply voltage for the rectifier cascade- GKand thus also the influencing of the control quantity which'can be tappedat the potentiometer R can be varied for the supply voltage.

Other changes and modifications of the invention may become apparent to those skilled in the art without departing from the spirit and scope of the invention and I therefore intend to include within the patent warranted hereon all such changes and modifications as may reasonably and properly'be included within the scopeof my contribution to the art.

I claim:'

l; A circuit arrangement for the control of the output voltage of a high voltage power supply unit which has an output circuit including a rectifier cascade, comprising a voltage divider including a high ohmic resistor and a potentiometer connected in series with said resistor, said resistor connected to a point in said rectifier cascade and said potentiometer connected to a reference potential to provide a low voltage representation of the high output voltage of the rectifier cascade, said potentiometer including a movable tap, an amplifier having an input connected to said movable tap and an output, a difference amplifier having a first input connected to the reference potential, a second input connected to said output of said amplifier, and an output connected to the power supply unit for feeding a control signal thereto, and a Zener diode connected between the output of said difference amplifier and the reference potential to limit the control of the power supply unit by the output of said difference amplifier. 

1. A circuit arrangement for the control of the output voltage of a high voltage power supply unit which has an output circuit including a rectifier cascade, comprising a voltage divider including a high ohmic resistor and a potentiometer connected in series with said resistor, said resistor connected to a point in said rectifier cascade and said potentiometer connected to a reference potential to provide a low voltage representation of the high output voltage of the rectifier cascade, said potentiometer including a movable tap, an amplifier having an input connected to said movable tap and an output, a difference amplifier having a first input connected to the reference potential, a second input connected to said output of said amplifier, and an output connected to the power supply unit for feeding a control signal thereto, and a Zener diode connected between the output of said difference amplifier and the reference potential to limit the control of the power supply unit by the output of said difference amplifier. 